The present invention relates generally to electromagnetic interference (xe2x80x9cEMIxe2x80x9d) and more particularly to shielding electronic components from EMI and radio frequency interference (xe2x80x9cRFIxe2x80x9d).
The acronym xe2x80x9cEMIxe2x80x9d refers, typically, to unwanted electrical xe2x80x9cnoisexe2x80x9d that leaks, in the form of magnetic fields, from a power-carrying line. Noise signals that act over a substantial distance are usually referred to as RFI. EMI/RFI (hereinafter simply xe2x80x9cEMIxe2x80x9d) may adversely affect the functioning of electronic equipment. The levels of EMI reaching such electronic equipment must therefore be reduced to benign levels.
Typical EMI countermeasures include a combination of filtering, for conducted EMI (i.e., noise transmitted through wires), and shielding, for radiated EMI. To provide maximum protection for radiated EMI, shielding methods are implemented at more than one xe2x80x9cassemblyxe2x80x9d level. For example, shielding may be provided at the component level, the printed circuit board (xe2x80x9cPCBxe2x80x9d) level, the shelf level or the cabinet level, or a combination thereof as appropriate.
Shielding may be particularly important or critical at one specific assembly level as a function of the electronic device being considered. For example, PCB-level shielding is particularly important for wireless terminals (e.g., cellular telephones, cordless telephones, cellular modems, etc.).
In the context of wireless terminals, PCB-level shielding arrangements/methods that are known in the prior art fall into one of several categories. Those categories include:
(1) permanently PCB-mounted metal shielding xe2x80x9ccans;xe2x80x9d
(2) permanently PCB-mounted metal wall (fence) with removable clip-on cap;
(3) removable plastic (conductor-coated or plated) shielding cans with or without conductive elastomeric gasket;
(4) removable metal cans with weld-on spring metal gasket strip; and
(5) housing-integrated shielding wherein inner surface of housing is conductor-coated and gasket is applied to edge of walls in contact with the ground track on the PCB.
While shielding solutions selected from these categories are expected to be effective at rejecting EMI, they all suffer from a variety of drawbacks. In particular, shielding falling into categories (1) and (2) suffer from air/heat flow problems that create difficulties in heating solder, thereby preventing the use of surface mounted chips. Moreover, shielding from categories (1) and (2) may have limitations pertaining to the size of the shielding, the co-planarity of the metal xe2x80x9ccan,xe2x80x9d and test and repair requirements.
Using removable shielding, as is embodied by shielding from categories 3-5, eliminates air/heat flow problems and is desirable for PCB testing and repair. Such shielding suffers, nevertheless, from other drawbacks. Specifically, shielding falling into categories 3 and 5 is expensive, requires a relatively large ground track width on the PCB, requires a specialized vendor to produce quality parts and uses materials/processing steps that raise environmental concerns. And shielding from category (4) requires a wide ground track, has a requirement that there be no internal section walls, and requires expensive welding techniques (i.e., to weld the tiny metal spring strip to the shielding can).
Notwithstanding the variety of shielding options available, a need therefore exists for improved PCB-level shielding.
The present invention provides PCB-level EMI shielding that avoids many of the drawbacks of the prior art. In particular, the present shielding is easy to manufacture, easy to install, easy to remove, is capable of shielding separate chambers, has a small form factor and is low in cost.
In one embodiment in accordance with the present teachings, the EMI shielding comprises a xe2x80x9ccan,xe2x80x9d xe2x80x9cboxxe2x80x9d or xe2x80x9cpartial enclosurexe2x80x9d that is open on one side. The can has a substantially flat planar base and walls that depend from the edges of the base. The can is formed from a material that is substantially opaque to EMI in at least the operating frequency range of the electronic components being shielded.
In some embodiments, the distal end of each wall (i.e., distal relative to the base) is bent toward the outside of the can thereby forming a xe2x80x9clipxe2x80x9d on each wall. Such lips collectively form a gasket-like member at the open end of the can. When the can is in use, the gasket-like member contacts a grounding element on a PCB, such as, for example, a ground track. In a typical application, the present shielding is disposed within an electronic device sandwiched between the PCB and a housing.
For PCBs requiring multiple shielding zones, a like number of the present shielding cans are suitably fabricated. To the extent that the device housing is compartmentalized to facilitate such multiple shielding zones, the present cans are advantageously preformed to fit into such compartments.
The gasket-like member at the open end of the can is advantageously physically adapted to provide a resilience or xe2x80x9cspringiness.xe2x80x9d In one embodiment, the resilience is imparted by the bend that is used to form the lip for each wall. Consequently, when the can is sandwiched in position between a PCB board on one side and a housing on another side, the resilience of the gasket-like member facilitates reliable electric contact between the can and the grounding element of the PCB. In further embodiments, the lip of each wall is further deformed (e.g., bent, cut, etc.) to provide additional functionality (e.g., a channel for receiving ribs of the housing) or for further improving conformance of the lip to the ground track (e.g., a plurality of spring members formed within the lip).